While the causes of the decline of the Indus Valley civilization are not clear, it is possible that the periodic shifts in the courses of the major rivers of the valley may have deprived the cities of floodwaters necessary for their surrounding agricultural lands. The cities thus became more vulnerable to raiding activity. At the same time, Indo-Aryan peoples were migrating into the Indian subcontinent through the northwestern mountain passes, settling in the Punjab and the Ganges valley.

Over the next 2,000 years the Indo-Aryans developed a Brahmanic civilization , out of which Hinduism evolved. From Punjab they spread east over the Gangetic plain and by c.800 BC were established in Bihar, Jharkhand, and Bengal . The first important Aryan kingdom was Magadha , with its capital near present-day Patna; it was there, during the reign of Bimbisara (540-490 BC), that the founders of Jainism and Buddhism preached. Kosala was another kingdom of the period.  
In 327-325 BC, Alexander the Great invaded the province of Gandhara in NW India that had been a part of the Persian empire. The Greek invaders were eventually driven out by Chandragupta of Magadha, founder of the Mauryan empire. The Mauryan emperor Ashoka (232 BC), Chandragupta's grandson, perhaps the greatest ruler of the ancient period, unified all of India except the southern tip.